Receive the latest national-international updates in your inbox

Investigators in white jumpsuits work the crime scene on Boylston Street following Monday's bomb attack at the Boston Marathon.

Authorities investigating the Boston Marathon bombings have the face but not the name of a person they have spotted on surveillance video leaving a black bag near the scene of the blasts, NBC News reported.

The person was seen on a nearby Lord & Taylor department store's surveillance camera, an official told NBC News, as investigators pored over hours of video and a glut of photographs from the scene.

The development — less than 48 hours after the attack that left three people dead and more than 170 wounded — marked a possible turning point in a case that has investigators analyzing photos and videos frame by frame for clues to who carried out the twin bombings and why.

But the Federal Bureau of Investigation, Boston police, the U.S. attorney in Boston, a senior White House official and NBC News sources all agreed that no arrests have been made, despite reports in some media outlets to the contrary.

"Since these stories often have unintended consequences, we ask the media, particularly at this early stage of the investigation, to exercise caution and attempt to verify information through appropriate official channels before reporting," the FBI said in a statement.

Authorities told NBC News that they have video of several individuals, including the one captured on the Lord & Taylor camera, whom they are interested in questioning.

Senior officials told NBC News it is too soon to characterize individuals as suspects or witnesses. "We are zeroing in on some people," they said.

By nightfall Wednesday, at least 14 patients remained in critical condition. Dozens of patients have been released from hospitals around the Boston area, and officials at three hospitals that treated some of the most seriously injured said they expect all their remaining patients to survive.

Earlier in the afternoon, employees were evacuated from the Boston courthouse Wednesday afternoon after a bomb threat was called in, Boston police told NBC Connecticut. Police began allowing employees back inside about an hour later, according to NBC News reporters on the scene.

The developments came after investigators in white jumpsuits fanned out across the streets, rooftops and awnings around the blast site in search of clues. They picked through trash cans, plastic cup sleeves and discarded sports drink dispensers.

Amid that effort, authorities recovered a piece of circuit board that they believe was part of one of the twin explosive devices, and also found the lid of a pressure cooker that apparently was catapulted onto the roof of a nearby building, a law enforcement official briefed on the investigation told The Associated Press on Wednesday.

Law enforcement agencies had pleaded Tuesday for the public to come forward with photos, videos or any information that might help them solve the twin bombings that killed three people and wounded more than 170 a day earlier. Investigators circulated information about the bombs, which involved kitchen pressure cookers packed with explosives, nails and other lethal shrapnel — but the FBI said nobody had claimed responsibility.

"Someone knows who did this," Richard DesLauriers, FBI agent in charge in Boston, said Tuesday.

President Barack Obama branded the attack an act of terrorism but said officials don't know "whether it was planned and executed by a terrorist organization, foreign or domestic, or was the act of a malevolent individual." Obama plans to attend an interfaith service Thursday in the victims' honor in Boston.

Tyler King, a personal trainer from Attleboro who works in Boston, said four of five clients canceled on him a day earlier because they were worried about venturing into the city. He took the train in, but "I kind of kept my head on a swivel."

Kenya Nadry, a website designer, took her 5-year-old nephew to a playground. "There's still some sense of fear, but I feel like Boston's resilient," she said. "The fine men in blue will take care of a lot of it."

Police were stationed on street corners across downtown Boston, while National Guardsmen set up tents on the Boston Common and stationed tactical vehicles.

Dr. Horacio Hojman, associate chief of trauma at Tufts Medical Center, said patients were in surprisingly good spirits when they were brought in.

"Despite what they witnessed, despite what they suffered, despite many of them having life-threatening injuries, their spirits were not broken," he said. "And I think that should probably be the message for all of us — that this horrible act of terror will not bring us down."

President Barack Obama planned to visit Boston on Thursday to attend a service honoring the victims.